When a metal is introduced in seawater corrosion occurs on its surface. The corrosion on the metals, such as steel alloys, in determinates environments causes the apparition of gelatos biofilm formed just on the surface that is directly related with this process. This is possible only if the corrosion is carried out in a biotic environment, and if not will appear some solids that will be organics and inorganic substances products of the corrosion. Therefore, the behaviour of steel in natural environments depends on the material properties on the environment and on the microorganisms than can exist within it. In this research a specimen of carbon steel was introduced in two different environments, natural seawater and synthetic seawater. It had been examined with the help of a combination of different techniques: weight loss measurement, optic microscope test and electrochemical measurement. The results obtained with these techniques have shown that in a natural seawater a heterogeneous layer is formed by a mixture of corrosion products and biofilm that grew on the material surface, producing the increase of the corrosion rate the first periods of time and then a decrease in the corrosion resistance under diffusion process. On the other hand, in the synthetic seawater, there is not biofilm. In this case it is appeared organic and inorganic corrosion products. Moreover, some other factors are important in the corrosion. The pressure is an important factor that changes the behaviour of the metals in front of the corrosion. If the pressure is high the rate corrosion is slower. Therefore in greater depth, the effect of corrosion is less important. But the effect is more important in natural seawater than in synthetic seawater. The intelligent used of the techniques mentioned above allows the interpretation of the effect of the marine biofilm on the behaviour in carbon steel.

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